pragma solidity ^0.8.13;

contract ReceiptAnchor {
    address public immutable agent;
    mapping(bytes32 => uint256) private commitments; // 0 = never used, block.timestamp = active, uint256.max = revealed

    event Committed(bytes32 indexed commitment, uint256 timestamp);
    event Revealed(bytes32 indexed commitment, uint256 timestamp, address revealer);

    constructor() {
        agent = msg.sender;
    }

    function commit(bytes32 commitment) external {
        require(msg.sender == agent, "only agent");
        require(commitments[commitment] == 0, "already used");
        commitments[commitment] = block.timestamp;
        emit Committed(commitment, block.timestamp);
    }

    function reveal(bytes memory receiptPayload, bytes memory secret) external {
        bytes32 commitment = keccak256(abi.encode(receiptPayload, secret));
        uint256 ts = commitments[commitment];
        require(ts != 0 && ts != type(uint256).max, "not active");
        commitments[commitment] = type(uint256).max;
        emit Revealed(commitment, ts, msg.sender);
    }
}